In combustion processes, especially in grate-fired furnaces, the thermal formation of nitrogen oxide (NO, formation) from nitrogen in the air is negligibly small due to the relatively low temperature level. When fuels containing nitrogen are burned in these furnaces, nitrogen oxides are formed largely from the nitrogen bonded in the fuel.
The burn-out of solid fuels such as waste, biomass or coal on combustion grates can be divided in an idealized manner into the consecutive partial processes of drying, de-gassing and burn-out of the solid carbon. In industrial grate-fired furnaces, these partial processes overlap. During the de-gassing phase, not only the hydrocarbons but also the nitrogen compounds—especially NH3 (ammonia) and HCN (hydrogen cyanide)—that are formed primarily from the fuel nitrogen are released into the offgas. The concentration of hydrocarbons in the offgas directly above the grate, particularly in the area of the main combustion zone of the incineration system, is so high that the amount of oxygen fed locally there via the primary air is not sufficient to bring about a complete burn-out of the offgas. The offgas exiting from the combustion bed in this zone has high offgas temperatures and is practically oxygen-free. Under these conditions, carbon monoxide (CO) and hydrogen (H2) are formed via gasification reactions. Consequently, it is also in this area that the highest concentrations are found of high-heating-value offgas components such as hydrocarbons, carbon monoxide or hydrogen, along with the nitrogen species that are formed primarily from the fuel nitrogen, mainly ammonia (NH3) and hydrogen cyanide (HCN) as well as, in much smaller quantities, organic compounds containing nitrogen such as, for instance, pyridine and aniline.
Normally, in the case of the above-mentioned incomplete combustion due to a lack of oxygen, after-burning is initiated by adding secondary air to the still high-heating-value offgas. This gives rise to very high temperature peaks locally, whereby NO or N2 are ultimately formed from the above-mentioned NH3 and HCN compounds under the oxidizing conditions via complex reactions during the offgas burn-out. The objective is to modify the control of this process in such a way that the primary nitrogen species NH3 and HCN are completely degraded and that N2 is preferably formed as the final product, at the expense of the formation of nitrogen oxide while, at the same time, avoiding the formation of N2O.
M. V. A. Horttanainen, J. J. Saastamoinen, P. J. Sarkomaa: Ignition Front Propagation in Packed Bed of Wood Particles; IFRF Combustion Journal, Article No. 200003, May 2000, ISSN 1562-479X, describes the dependence of the burn-out rate on the primary air volume during the burn-out of solids. Depending on the properties of the fuel, particularly on the heating value, the burn-out rate displays a maximum at a certain primary air volume. A further increase in the primary air volume beyond this maximum, in contrast, causes the combustion bed to cool down. The reduced or delayed release of volatile fractions from the fuel that is associated with the cooling as well as the dilution of the combustion gases with the fed-in primary air cause a locally reduced release and thus a diminished concentration of hydrocarbons, CO and H2. H. Hunsinger, K. Jay, J. Vehlow: Formation of Pollutants during Municipal Solid Waste Incineration in a Grate Furnace under Different Air/Fuel Ratios; Proc. IT3 '02 Conference, May 13-17, 2002, New Orleans, La. and H. Hunsinger, J. Vehlow, B. Peters, H. H. Frey: Performance of a Pilot Waste Incinerator under Different Air/Fuel Ratios; IT3 '00 Conference, May 8-12, 2000, Portland, Oreg., both of which describe this measure in conjunction with the additional information that a high feed of primary air with a concurrent low feed of secondary air (constant sum of primary and secondary air) fundamentally lead to low NO values in the combustion offgases.
The injection of water for purposes of reducing the formation of PCDD/F in waste incineration plants is proposed in U.S. Pat. No. 5,313,895. An advantage is postulated to be the reduction in NOx formation due to the temperature drop caused by the injection of water. All of the offgas temperatures cited in U.S. Pat. No. 5,313,895 refer to the area before the gas enters the offgas burn-out zone and said temperatures are between 800° C. and 950° C. or 970° C. [1472° F. and 1742° F. or 1778° F.]. Unfortunately, however, detailed NOx values and NOx reduction rates are not given. Moreover, there is no information about other pollutants containing nitrogen, especially N2O and NH3.
Lowering the offgas temperatures to below 950° C. [1742° F.] after it has left the offgas burn-out zone, however, leads to incomplete degradation of the primarily formed NH3 (ammonia) and also to the formation of N2O (laughing gas), both of which escape into the atmosphere as strong greenhouse gases if they are not treated within the scope of additional process steps, for example, with catalysts.
The temperature reduction to 800° C. to 950° C. [1472° F. to 1742° F.] mentioned in U.S. Pat. No. 5,313,895 resulting from the addition of water, however, causes a drop in performance, even if the heat is utilized downstream, for instance, in order to heat up a boiler.
The same effect is achieved by moistening the fuel, which reduces the heating value of the fuel. The maximum of the burn-out rate is already exceeded at small primary air volumes. The burn-out of the solids extends over a long grate area, whereby the heating values of the gas as well as the offgas temperatures become established at a low level before the gas enters the offgas burn-out zone. The above-mentioned effects occur here as well.